Secondary batteries are devices capable of storing energy in chemical form and of converting into electrical energy to generate electricity when needed. The secondary batteries are also referred to as rechargeable batteries because they can be recharged repeatedly. Common secondary batteries include lead accumulators, NiCd batteries, NiMH accumulators, Li-ion batteries, Li-ion polymer batteries, and the like. When compared with disposable primary batteries, not only are the secondary batteries more economically efficient, they are also more environmentally friendly.
Secondary batteries are currently used in applications requiring low electric power, for example, equipment to start vehicles, mobile devices, tools, uninterruptible power supplies, and the like. Recently, as the development of wireless communication technologies has been leading to the popularization of mobile devices and even to the mobilization of many kinds of conventional devices, the demand for secondary batteries has been dramatically increasing. Secondary batteries are also used in environmentally friendly next-generation vehicles such as hybrid vehicles and electric vehicles to reduce the costs and weight and to increase the service life of the vehicles.
Generally, secondary batteries have a cylindrical, prismatic, or pouch shape. This is associated with a fabrication process of the secondary batteries in which an electrode assembly composed of an anode, a cathode, and a separator is mounted in a cylindrical or prismatic metal casing or a pouch-shaped casing of an aluminum laminate sheet, and in which the casing is filled with electrolyte. Because a predetermined mounting space for the electrode assembly is necessary in this process, the cylindrical, prismatic or pouch shape of the secondary batteries is a limitation in developing various shapes of mobile devices. Accordingly, there is a need for secondary batteries of a new structure that are easily adaptable in shape.
To fulfill this need, suggestions have been made to develop cable-type batteries having a very high ratio of length to cross-sectional diameter. The cable-type batteries are easy in shape variation, while being subject to stress due to external force for the shape variation. Also, the electrode active material layer of cable-type batteries may be released by rapid volume expansion during charging and discharging processes. From these reasons, the capacity of the batteries may be reduced and the cycle life characteristics thereof may be deteriorated.
Such a problem may be solved in a certain degree by increasing the amount of a binder used in the electrode active material layer to provide flexibility during bending or twisting. However, the increase of a binder amount in the electrode active material layer causes an electrode resistance rise to deteriorate battery performances. Also, when intense external forces are applied, for example, in the case that electrodes are completely folded, the release of the electrode active material layer cannot be prevented even though the amount of a binder becomes increased. Therefore, this method is insufficient to solve such problems.